Evaporation apparatus for evaporation of liquids

ABSTRACT

An evaporation apparatus for evaporation of, for example, spent liquor from pulp cooking. The apparatus has a vertical cylinder that has evaporation surfaces disposed therein. An outlet opening is defined in a widened part of the cylinder to receive driven-off steam. A skirt is disposed in the cylinder and surrounds the evaporated liquid and driven-off steam and has a diameter that is smaller than a diameter of the widened part. The skirt has an open bottom and a skirt opening is defined in a portion of the skirt that is facing away from the outlet opening.

TECHNICAL FIELD

The present invention relates to an evaporation apparatus forevaporation of liquids, especially liquids such as spent liquor frompulp cooking or other liquids within the pulping industry, whichevaporation apparatus consists principally of a vertical cylinder inwhich evaporated liquid is collected in the basin-shaped bottom part andvaporized steam is removed in the cylinder wall slightly above the poolof liquid in the basin-shaped bottom part. The invention relates inparticular to an arrangement for separating liquid droplets fromvaporized steam.

BACKGROUND INFORMATION AND SUMMARY OF THE INVENTION

Evaporation apparatuses for evaporation of liquids, for making thesemore highly concentrated or for separating solid substances fromsolvents, are generally known and are used generally within the chemicalindustry. In the pulping industry, such evaporators are used inparticular for evaporating the spent liquor from pulp cooking, whichspent liquor has to be concentrated by removal of water so that it canbe combusted in a later stage, for example in a recovery boiler. Theseevaporation apparatuses consist principally of a vertical plate cylinderwith vertically disposed tubes through which the liquid intended forevaporation is made to flow, while steam at higher temperature andpressure is made to flow in the opposite direction and condense on theoutside of the tubes. The necessary heat for evaporation of the liquid,in the case of spent liquor from the pulping industry, is thus suppliedthrough the tube walls. It is customary for the liquid intended forevaporation to be made to run downwards and for the heating steam toflow upwards in separate systems. The evaporated liquid thus falls downinto the bottom of the cylinder-shaped evaporation apparatus, whichbottom is usually slightly widened, and the vaporized steam is removedat the bottom part slightly above the pool of liquid which is formed inthe preferably basin-shaped bottom part by evaporated liquid.

There are many different constructions of such evaporation apparatuses.Thus, evaporators can be arranged for motive steam to flow through theinside of the tubes while the liquid to be evaporated is made to rundown the outside. Horizontally disposed tubes may also be provided, aswell as plates whose surfaces are made to serve as evaporation surfaces.

In the pulping industry it is customary to evaporate black liquor inseveral stages, by series or parallel coupling of several evaporationapparatuses. The units, which are called effects, are numbered inaccordance with the steam supply in the installation. Thus, fresh steamis supplied to the first effect, and in the subsequent stages the liquorsteam which has been obtained in preceding stages is used as a heatsource. This is possible by means of the successive reduction inpressure. Liquor steam from the last effect condenses in one or moresurface condensers. By allowing the liquor steams to condense out instages on different heat surfaces in the installation, it is possible toseparate off, on the one hand, highly contaminated condensate forpurification in, for example, a stripper column, and, on the other hand,purer condensate which can be used directly in the mill without odourtreatment or other purification.

At the present time, ever greater demands are being made in mostcountries for a cleaner environment. Water which is to be dischargedinto rivers and the like must be pure, or at least so pure that it doesnot cause environmental problems. It is no longer possible to dischargesemi-pure condensate. One way of reducing discharges could be for thecondensate to be re-used as process water in the mill.

Insufficiently careful separation of evaporated liquid from vaporizedsteam, by means of small or large droplets being entrained with thesteam, so-called liquor carry-over, which gives the condensate formed abrownish character, means that the condensate cannot be returned withoutfurther cleaning to the fibre line, e.g. for bleaching. The separationof the evaporated liquid from vaporized steam must therefore bevirtually complete if the condensate is to be re-used as process waterin the last stages of the fibre line, without impairing the quality ofthe final product, the paper pulp.

According to the prior art, a vertical cylindrical space without abottom has therefore been arranged under the evaporation surfaces, bymeans of which the steam and the droplets are made to flow downwards,the larger droplets settling on the underlying bottom of the evaporationapparatus, and the small droplets being carried along in the flow ofsteam which is deflected upwards and separated in a droplet separatorfor vertical approach flow. After this separation of the small dropletswhich come together in the droplet separator and run down into the poolof liquid, the purified steam is led out from the effect.

Since the steam flowing upwards on the outside of the cylindrical spacewill have different speeds in different parts of the annular spaceoutside the skirt, the droplet separation will also be non-uniform. Thespeed of the steam will be greatest nearest the outlet opening, and atthe diametrically opposite side of this it is possible to have very lowspeeds and even downward flows through the droplet separator. There istherefore a considerable need for an even approach flow of vaporizedsteam towards the droplet separator and a consequently better separationof the droplets, with concomitant reduction in the cost of theconstruction.

According to the present invention, the above-mentioned problem ofcarry-over of liquid droplets has therefore been solved by makingavailable an evaporation apparatus for evaporation of liquids, such asspent liquor from pulp cooking, comprising a vertical cylinder withinlet and outlet openings for steam and liquid, and evaporation surfacesin the cylinder, the apparatus being arranged to generate a downwardflow of evaporated liquid and driven-off steam, which is characterizedin that the outlet opening for driven-off steam is situated in the lowerand preferably widened part of the cylinder, and in that the lower partof the space under the evaporation surfaces, the “skirt”, surroundingthe evaporated liquid and driven-off steam, is cylindrical with asmaller cross-sectional diameter than the lower part of the cylinderwhich has the outlet opening for steam, and in that the skirt, inaddition to having an open bottom, also has an opening for outflowingsteam on that part of the skirt which faces away from the outlet openingin the outer cylinder.

According to the invention, it is expedient for the opening in the skirtto be made by means of the latter having been cut obliquely downwards.

It is however possible, according to the invention, for the opening inthe skirt to be made by means of its having been cut in a stepped shapedownwards.

According to the invention, the skirt, with its lower edge not cut away,is intended to extend down into the underlying pool of evaporated liquidin the bottom part of the cylinder.

According to the invention, it is expedient for the skirt to be curvedinwards at its upper part.

According to the invention, droplet separators for horizontal approachflow of steam are expediently arranged, in the direction of movement ofthe steam, in front and on both sides of the outlet opening.

According to the invention, the droplet separators can consist of two ormore sections arranged one above the other.

According to the invention, the upper section or sections of the dropletseparators are expediently arranged, in the approach flow direction ofthe steam, in front of the underlying section or sections.

According to the invention, drainage systems for liquid emptying intothe pool of liquid in the bottom part of the cylinder are expedientlyarranged in each section of the droplet separator.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in greater detail hereinbelow withreference to the attached figures, of which:

FIG. 1 shows a vertical section of the lower part of an evaporatoraccording to the prior art,

FIG. 2 shows a diagrammatic representation of an evaporator according tothe present invention,

FIGS. 3A and 3B show a droplet separator arrangement according to theinvention, seen from above and from the side,

FIG. 4 shows a further embodiment of the present invention, and

FIGS. 5A and 5B show a second embodiment of the separator.

DETAILED DESCRIPTION

FIG. 1 shows the lower part of a vertically disposed, cylinder-shapedevaporator 1 of a known type, with a widened lower part 2, and the lowerpart of the tube assembly 3 where the actual evaporation takes place.The space below the evaporation tubes 3 is delimited by acylinder-shaped so-called skirt 4, open at the bottom, through whichvaporized steam and evaporated liquid in the form of droplets are madeto flow downwards. The droplets are collected together in the lower part2 of the cylinder 1 in the form of a pool 6, and the vaporized steam ismade to flow out through an outlet 7. From the lower part 2,arrangements (not detailed) are made for removing liquid from the pool 6of liquid. The large liquid droplets fall like rain down into the pool 6of liquid, while the smaller ones have a marked tendency to be entrainedupwards with the steam as it is deflected at the skirt edge. A dropletseparator 8 is therefore arranged for collecting these small droplets.This droplet separator 8 has a vertical approach flow. Dropletseparators per se are well known; they consist, for example, of anassembly of bent deflection plates, and are not described in detailhere.

The ability of the droplet separator to separate droplets from the gasflow is highly dependent on the speed of the gas flow. The greater thegas flow, the poorer the droplet separation. It is therefore importantthat the speed of the gas flow approaching the droplet separator isuniform in all parts. However, the way this arrangement according to theprior art is made, the flow approaching the droplet separator does nothave a uniform speed everywhere, and instead the speed will be greaterthe nearer to the outlet opening 7 the gas flow runs through the dropletseparator 8. On that side of the widened bottom part 2 which liesdiametrically furthest away from the outlet opening 7, the flow of steamwill have its lowest speed, and it may even be possible for there to bea downward flow of steam.

In order to ensure that a droplet separator according to the prior artwill be able to function, the lower part 2 of the apparatus must ofcourse be made very large so that a sufficiently low speed of steam isobtained near the outlet 7. The droplet separator 8 is otherwiseoverloaded in the region nearer the outlet opening 7 and liquor dropletsare entrained with the outflowing steam.

FIG. 2 shows diagrammatically how an evaporation apparatus 1 accordingto the present invention is made. It looks in principle like the one inFIG. 1, but it has a modified skirt 4. The skirt 4 has, however, anopening outwards to the left in the figure, which opening has beenproduced by means of the skirt 4 having been cut off obliquely at theedge 9. The skirt protrudes, via its part not cut off, into the pool 6of liquid. The previous droplet separator 8 with vertical approach flowhas been replaced, according to the invention, by droplet separators 10with horizontal approach flow. These droplet separators 10 are notnecessary per se in all cases. For example, the outflowing steam can beconveyed in co-current with the liquor in the tubes in anotherevaporation apparatus. This affects the downwardly running film ofliquor (more turbulence) so that a higher heat transfer is obtained.

The steam/liquid mixture flowing downwards inside the skirt will bedeflected in a radial direction outwards as it flows down through theskirt and then deflected in a tangential direction past the skirt edge 9and towards the droplet separators 10. The direction of flow is in thisway changed in two stages. First from an axial direction downwards to aradial direction outwards, and then a tangential direction in the spacebetween the skirt and the jacket wall of the widened part 2 of thecylinder 1. The liquid droplets are in this way flung slightly outwardswhen deflection occurs and fasten to a wall surface, and the dropletseparators 10 acquire a uniform horizontal approach flow. In this way itis possible to build the lower part 2 of the evaporation apparatus 1smaller, since the approach flow towards the droplet separators 10 is toall intents and purposes uniform.

FIG. 3A shows a horizontal section and FIG. 3B a vertical sectionthrough the lower part 2 of the cylinder 1 which is shown in FIG. 2.There are two droplet separators 10 on each side of the outlet opening7. These droplet separators 10 close off the whole annular space betweenthe skirt 4 and the cylinder wall 2 in the lower widened part of theevaporation apparatus, for which reason all the steam has to flowthrough these droplet separators 10. In the present case, the dropletseparators 10 are mounted upright and with relatively small thickness,but other types of droplet separators may also be used.

For the sake of clarity, the lower liquor space can be divided intothree flow regions.

I. Inside the diagonally cut skirt.

II. The annular corridor outside the skirt, before the horizontallyapproached droplet separators.

III. The annular corridor outside the skirt, after the dropletseparator, before the outlet.

The design of the lower liquor space creates a flow which ischaracterized by the following.

1. The flow of steam, as it leaves the tube plate and passes into theregion I, is directed downwards for the whole cross-section. The flowthus has only an axial speed.

2. The flow at the centre of region I continues axially downwards. Theaxial speed decreases and the radial speed increases.

3. The flow in those parts of region I which are open for transport intoregion II has a high radial speed.

4. The flow, from region I which reaches the skirt edge, changesdirection when it passes into region II. The speed becomes tangential.

5. This change in the direction of flow occurs successively in the axialdirection.

6. The flow, when it leaves region II, has a uniform tangential speedtowards the droplet separator.

7. The flow, when it passes into region III, again changes direction andaccelerates towards the outlet.

By designing the lower liquor space in the manner described, a flow isgenerated which is very near the optimum. The optimum design gives thefollowing results.

1. A maximum separation of droplets occurs even before the dropletseparator, since the steam is twice deflected at right angles from itsoriginal direction of movement (only very small droplets with extremelysmall mass manage to remain with the steam). In this way, the loading ofthe droplet separator is minimized.

2. The approach flow to the droplet separator has a uniform speedprofile and the total approach flow surface of the whole dropletseparator is utilized. The degree of separation by the droplet separatoris maximized in this way.

3. Separated liquid in the droplet separator may sometimes beconcentrated into large droplets which can be carried along with thesteam downstream of the droplet separator, a problem which is referredto as entrainment. The uniform speed profile, however, means that thespeeds downstream of the droplet separator are minimized. This alsominimizes the risk of these larger droplets being carried along with thesteam out of the evaporation apparatus, and instead they fall downwards.

FIG. 4 shows another embodiment of the present invention. Here, theskirt 4 is curved inwards at the part 11, and the skirt edge 12 has beencut with an upper diagonal edge and a lower vertical edge. The upperpart of the annular space between the skirt 4 and the lower part 2 ofthe evaporation apparatus is in this case larger, which affords a morefavourable, slower flow towards the outlet 7 and the possible dropletseparators 10.

FIGS. 5A and 5B, finally, show a two-part form of the droplet separator.This is shown seen from the side in FIG. 5A and through section A—A inFIG. 5B. The droplet separator consists of an upper part 13 and a lowerpart 14. The approach flow of steam towards the droplet separators 13and 14 is in the direction of the arrow from the right and, as can beseen from the figure, the upper part 13 is slightly more displacedtowards the flow direction than the lower part 14. Between the dropletseparators 13 and 14 there is a partition wall 15, so that the steamwill not be able to flow past the separators. Drainage tubes 16 arearranged extending from each droplet separator down to the pool 6 ofliquid.

To ensure that steam does not slip past the droplet separators when thesurface of the pool 6 of liquid reaches too low a level, a horizontalplate is arranged between the lower part of the skirt and the jacketwall in the lower part 2 on the underside of the droplet separator 14.

The dimensions of the skirt and the lower part 2 of the cylinder 1 canvary, but the outer lower part 2 of the cylinder 1 should have a radiuswhich is 25-200% greater than the radius of the skirt 4, preferably50-100% greater.

The flow conditions which are customary in the evaporation apparatusaccording ro the present invention, when it is being used forevaporation of black liquor, are an absolute pressure of 1 -15 bar,preferably 0.07-4 bar, a temperature of 0 -250° centigrade, preferably40-145° centigrade, and a steam specific volume of 0.12-40 m³ of steamper kilo of steam, preferably 0.5-20 m³ of steam per kilo.

The evaporator has been described as a cylinder with a roundcross-section, but a cross-section in the form of a polygon is alsoconceivable. A stepwise deflection of the flow of steam at the outerwall may in this case be beneficial in separating the droplets.

The invention is not limited to the embodiments which have been shown,and instead it can be varied in different ways within the scope of thepatent claims.

What is claitned is:
 1. An evaporation apparatus for evaporation ofliquids from pulp cooking, comprising: a vertical cylinder having aninlet opening defined therein for receiving steam and evaporated liquid,the vertical cylinder having a lower part, the vertical cylinder beingadapted to generate a downward flow of evaporated liquid and driven-offsteam; an outlet opening defined in the lower part for receivingdriven-off steam disposed in the lower part; a plurality of evaporationsurfaces disposed in the vertical cyinder; and a skirt disposed belowthe evaporation surfaces, the skirt being cylindrical and having across-sectional diameter that is smaller than a cross-sectional diameterof the lower part, the skirt surrounding the evaporated liquid anddriven-off steam, the skirt having an open bottom and a skirt openingdefined in a portion of the skirt that faces away from the outletopening, the skirt opening being defined by an obliquely cut downwardedge.
 2. The evaporation apparatus according to claim 1 wherein theskirt opening is defined by an upper obliquely cut edge and a lowervertical edge.
 3. The evaporation apparatus according to claim 1 whereina pool of the evaporated liquid is disposed in a widened part and theskirt has a lower edge that extends into the pool.
 4. The evaporationapparatus according to claim 1 wherein the skirt has an inwardlyinclined upper part.
 5. The evaporation apparatus according to claim 1wherein the evaporation apparatus further comprises a pair of dropletseparators extending vertically in front of and on each side of theoutlet opening to horizontally receive the driven-off steam.
 6. Theevaporation apparatus according to claim 1 wherein droplet separatorseach has an upper section disposed above a lower section.
 7. Theevaporation apparatus according to claim 6 wherein the upper section isdisposed upstream of the lower section.
 8. The evaporation apparatusaccording to claim 7 wherein the upper and lower sections each has adrainage system for draining liquid from the upper and lower sectionsinto a pool of the evaporated liquid disposed in the widened part. 9.The evaporation apparatus according to claim 1 wherein the lower part isa lower widened part.